1. Field of the Invention
The present invention relates generally to the field of photography. More specifically, the invention relates to an apparatus and method for electro-optical split image focusing.
2. Description of the Related Art
Conventional methods for finding a focal position of a photographing apparatus, such as a single-lens reflex (SLR) camera or a compact camera, include auto-focus methods whereby a photographing apparatus detects a focal position automatically by driving a focusing lens, and manual focus methods whereby a user drives a focusing lens manually while inspecting a view finder or a screen.
In manual focusing methods, a subject image is enlarged and displayed as a live view on a screen so that a focal position of the photographic apparatus can be inspected by the user. Auto-focus methods generally rely upon other techniques, such as a phase difference detection method or a contrast detection method (also known as video auto-focusing).
The phase difference detection method is generally used in an SLR camera. In the phase difference detection method, a prism is disposed in a portion of a photographing device on which light is incident. The prism splits the image from the subject, so that an amount of deviation in the image plane may be detected, and the position of the focusing lens adjusted to minimize the amount of deviation. Other methods are known for generating a split image. For example, a split image may be generated by disposing a liquid crystal diffraction grating in a portion of a photographing device on which light is incident.
The contrast detection method is generally used in a compact camera. In the contrast detection method, as a focusing lens is moved, image information is obtained. The position of the focusing lens is determined by moving the focusing lens to a position in which contrast is highest (i.e., a position in which the largest amount of edges is detected).
Conventional methods suffer from several different problems. For example, when the photographing device is supported by hands, especially when the view finder or screen is not enlarged, unsteady hands can make it difficult to see a view finder or screen, and thereby determine a focal position manually. In addition, even when a focusing lens is manipulated based on an enlarged display, it is difficult to check deviation of a focal position with naked eyes. These problems also make focusing difficult to perform fast.
Conventional auto-focus methods also suffer from problems. For example, when the contrast detection method is used for auto-focusing, the focusing lens is driven in a progressive scan, and detection of the focal position in the middle of scanning is not required. Thus, it can take more time to determine focal position using the contrast detection method. In addition, in the compact camera, there is typically no space for placing a prism in an optical path, making it difficult to use the phase difference detection method in the compact camera.
To save space, a liquid crystal diffraction grating has also been used with the phase difference detection method. However, the response speed of liquid crystals is typically slow, and a time lag may prevent proper focus from being achieved during photography. In addition, liquid crystal diffraction gratings are not well-adapted for use with split image focusing. Typically, liquid crystal diffraction gratings have alternately disposed layers of high density liquid crystal particles and low density liquid crystal particles. When such a layered liquid crystal diffraction grating is fixed at a predetermined angle, the refractive index is approximately uniform. Thus, when an aperture of a lens is greater than a predetermined diameter (which is a function of the refractive index of the diffraction grating), edges of the image are cut-off, and split image focusing becomes impossible.